1. Field of Invention
The invention disclosed in this document is an illuminator that outputs linearly polarized light. More particularly, the illuminator is suitable for incorporation in LCoS microdisplay based light engines of the type used in video projector applications.
2. Discussion of Background
A block drawing of a set of components 100 behind the screen of a LCoS microdisplay based rear projection video display (such as a TV) is illustrated in FIG. 1. To the left of FIG. 1, the light source 102 and condenser 104 elements are collectively called the illuminator 110. FIG. 1 also illustrates a kernel 120 projection lens 130 fresnel lens and projection screen 140. The kernel 120 includes light management system including LCoS microdisplays 122A-122C.
The light source 102 is usually a mercury short arc lamp. This type of lamp emits an output light beam that is round and has a spatially non-homogeneous intensity profile. One function of the condenser 104 is to shape and size the output light beam 106 to match that of the rectangular microdisplays 122A-122C of the kernel 120 and to homogenize the light intensity within the output light beam 106. Other functions of the condenser 104 are to set the f# of the illuminator output light beam 108 and to remove the infrared and the ultra violet portions of the light source emission spectrum. Some condensers 104 also remove undesired portions of the visible spectra from the illuminator output light beam 108. (A common example of this function is the removal of the “yellow” spike from the emission spectra produced by the mercury short arc lamp.) Additional requirements placed on the illuminator 110 are that it be physically compact and inexpensive.
The subject of this disclosure relates to the polarization of the output light beam 106 from the illuminator 110. The lamp 102 itself outputs unpolarized light. In many applications the illuminator output light beam 108 is also unpolarized light and this is acceptable. Other applications require that the illuminator output light beam 108 is linearly polarized light.
There are a number of conventional techniques known to the art by which to produce an illuminator 110 that outputs linearly polarized light. Three of the more widely used techniques are as follows:                The inclusion of a linear polarizer in the illuminator 110. The polarizer is typically a PBS or a reflective polarizer. In either case, the big disadvantage of this approach is the loss of a minimum of half the light produced by the illuminator 110.        The inclusion of a so-called “fly's eye” recombination system into the illuminator 110. Such an illuminator 110 includes a lamp 102 with a parabolic reflector 103, a fly's eye lens, a polarization rotator array plate and a second fly's eye lens. This type of system requires careful alignment and is quite expensive.        The inclusion of an integrating rod based recombination system into the illuminator 110. Such an illuminator 110 includes a lamp 102 with an elliptical reflector and a modified integrating rod. The modification consists of the addition of a reflective aperture and a quarter waveplate at the input end of the rod and the addition of a reflective polarizer at the rod output. The resulting illuminator 110 certainly outputs linearly polarized light but the intensity is found to be only a modest increase over the approach of simply inserting a polarizer into the illuminator 110.        